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To begin to display a film, a thin membrane that can develop on the surface of some things. Ew, don't eat that soup—it's been sitting out so long that it's filmed over.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for something] to develop a film on its surface. The windows had filmed over because of all the humidity. Her eyes filmed over with the cold.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.